Republican Nikki Haley has suspended her campaign, ensuring that Donald Trump will win the party’s nomination and again face Democratic President Joe Biden in November’s US election.

Haley bowed out on Thursday morning (AEDT) but did not endorse Trump, who she said must now try to win the backing of her supporters, who include moderates and independent voters.

Haley’s decision came a day after Super Tuesday, when Trump beat her soundly in 14 of the 15 Republican nominating contests.

“It is now up to Donald Trump to earn the votes of those in our party and beyond it who did not support him. And I hope he does that,” she said.

“At its best, politics is about bringing people into your cause, not turning them away and our conservative cause badly needs more people.”

Haley lasted longer than any other Republican challenger to Trump but never posed a serious threat to the former president. His iron grip on the Republican party base remains firm despite his multiple criminal indictments.

The rematch between Trump, 77, and Biden, 81 – the first repeat US presidential contest since 1956 – is one that few Americans want. Opinion polls show both Biden and Trump have low approval ratings among voters.

The election promises to be deeply divisive in a country already riven by political polarisation. Biden has cast Trump as an existential danger to democratic principles, while Trump has sought to re-litigate his false claims that he won in 2020.

Haley, 52, had drawn support from deep-pocketed donors intent on stopping Trump from winning a third consecutive Republican presidential nomination.

She ultimately failed to pry loose enough conservative voters in the face of Trump’s dominance.

But her stronger showing among moderate Republicans and independents highlighted how Trump’s scorched-earth style of politics could make him vulnerable in the November 5 election.

On March 3, Haley won the Washington, DC, Republican primary with 62.9 per cent of the vote, versus 33.2 per cent for Trump. On Tuesday, her only win came in Vermont, a small, deeply Democratic state.

Biden has his own baggage, including widespread concern about his age. Three-quarters of respondents in a February Reuters/Ipsos poll said he was too old to work in government, after already becoming the oldest US president in history.

About half of respondents said the same about Trump.

The central issues of the 2024 campaign have already come into focus. Despite low unemployment, a red-hot stock market and easing inflation, voters have voiced dissatisfaction with Biden’s economic performance.

Biden’s other major weakness is the state of the US-Mexico border, where a surge of migrants overwhelmed the system after he eased some Trump-era policies. Trump’s hawkish stance on immigration – including a promise to initiate the largest deportation effort in history – is at the core of his campaign, just as it was in 2016.

Trump may be dogged by his myriad criminal charges throughout the year. The federal case charging him with trying to overturn the 2020 election, perhaps the weightiest he faces, has been paused while Trump pursues a long-shot argument that he is immune from prosecution.

Abortion, too, will play a crucial role after the nine-member US Supreme Court, buoyed by three Trump appointees, eliminated a nationwide right to terminate pregnancies in 2022. The subject has become a political liability for Republicans, helping Democrats over-perform expectations in the 2022 midterm elections.

Haley, a former governor of South Carolina, had been among the first Republican contenders to enter the race in February 2023. But she was largely an afterthought until garnering attention for her stand-out debate performances later in the year.

She put her foreign policy expertise at the centre of her campaign, adopting hawkish stances toward China and Russia and forcefully advocating for continued aid to Ukraine, a stance that put her at odds with the more isolationist Trump.

But she was reluctant to completely disavow her former boss – she was Trump’s United Nations ambassador – despite his four indictments and two impeachments. Trump showed no such reticence, frequently insulting her intelligence and Indian heritage.

Only in the last months of her campaign did Haley begin to forcefully hit back at Trump, questioning his mental acuity, calling him a liar and saying he was too afraid to debate her.